Deformation types in wire loading-unloading cycle

Steps:

• Examine PQ: force rises linearly to maximum at Q, indicating elastic behavior if straight line.
• Examine QR: force falls to zero at R; if R extension >0, permanent set shows plastic deformation overall.
• At Q, maximum extension under load represents elastic strain component, recoverable upon unloading.
• Distinguish: total deformation at Q includes elastic part, but unloading reveals plastic hysteresis.

Why C is correct:

• Elastic deformation is reversible strain under stress; at Q, the wire's extension follows Hooke's law (stress ∝ strain), confirming elastic nature before full unloading.

Why the others are wrong:

• A: PQ may obey Hooke's law, but graph likely shows nonlinearity or yield beyond, invalidating strict obedience.
• B: PR is not the actual path (it's PQ then QR); spring constant applies only to elastic segments, not hypothetical PR.
• D: Work done is area under PQ curve, not F × extension at P (likely zero extension, so incorrect).

Final answer: C